Stream Cipher and Block Cipher Confusion Diffusion Stream

- Stream Cipher and Block Cipher - Confusion & Diffusion

Stream cipher & Block cipher n n Symmetric cipher divided into two categories: ¨ Stream cipher ¨ Block cipher Stream cipher: convert plaintext data into cipher text one byte or one bit at a time. P: BITS Key: K Cipher: D C= Ek(U) n Example: Caesar cipher, monoalphabetic and vegenere cipher. 2

Stream cipher example n In text format Pay 100 XOR in binary format 010111001… 100101011 (key) 11001001 cipher text n Note: each byte of the plain text is encrypted one after the other. 3

Block Cipher Rather than encrypting one byte at a time, a block of bytes is encrypted at once. n The message is broken into blocks. n 4

Block Cipher • Divide input bit stream into n-bit sections, encrypt only that section, no dependency/history between sections

Block cipher n n FOUR AND FOUR encrypt Vfas y. Tzx Vfas Generally block size is 64 bits 6

Confusion & Diffusion n Claude Shannon introduced the concept of confusion and diffusion, which are significant from the perspective of the cryptographic technique. 7

Confusion n n Confusion is a technique of ensuring that a cipher text gives no clue about the original text. Attacker should not be able to reveal the plain text by changing the one character of cipher text. Confusion is achieved by means of the substitution techniques. The idea of confusion is to hide the relationship between the cipher text and the key If single bit of key is changed , most or all bits in the cipher text will also be changed.

Confusion n Example ¨ One time pad provide good confusion because one plain text letter can be transformed to any cipher text letter at different places in the output. ¨ Caesar cipher is not a good example of Confusion.

Diffusion n n Diffusion increase the redundancy of the plain text by spreading it across rows and columns. diffusion is achieved by means of the transposition techniques. Diffusion hide the relationship between the cipher text and plain text. In diffusion each character of cipher text is dependent on some or all characters in the plain text. n NOTE: ¨ Stream cipher relies only on confusion ¨ Block cipher uses both confusion and diffusion. n Example: Hill cipher is a good example of diffusion. 10